Dual-mode luminescent thermometry in LiYO<ce:inf>2</ce:inf>:Nd<ce:sup>3+</ce:sup>,Yb<ce:sup>3+</ce:sup> enabled by structural phase transition and phonon-assisted energy transfer

In this work, a dual-mode luminescent thermometer operating via both ratiometric and lifetime-based readout strategies was developed, enabled by the coexistence of two thermally driven effects: a structural phase transition in LiYO₂ and a phonon-assisted energy transfer from Yb³⁺ to Nd³⁺. As demonstrated, changes in the shape of the emission band of Yb³⁺ ions corresponding to the ²F_5/2 → ²F_7/2 electronic transition, induced by the phase transition, allowed the design of a ratiometric thermometer with a maximum relative sensitivity (<ani:em>S</ani:em> _{<ani:em>R</ani:em>}) of 3.1% K⁻¹ for LiYO₂:10%Yb³⁺, 1% Nd³⁺ at 290 K. On the other hand, the temperature-dependent Yb³⁺ → Nd³⁺ energy transfer facilitated the development of a lifetime-based thermometer with a maximum <ani:em>S</ani:em> _{<ani:em>R</ani:em>} of 1.5% K⁻¹ for 20% Nd³⁺ at 378 K. In both approaches, tuning the Nd³⁺ ions concentration enabled modulation of both the sensitivity and the temperature at which the maximum <ani:em>S</ani:em> _{<ani:em>R</ani:em>} was achieved. This was accomplished by shifting the phase transition temperature and enhancing the probability of interionic energy transfer, respectively. Notably, the temperature ranges corresponding to the maximum <ani:em>S</ani:em> _{<ani:em>R</ani:em>} for the ratiometric and lifetime modes were found to be distinct, effectively broadening the thermal operating range of the sensor. Additionally, it was shown that LiYO₂:Nd³⁺, Yb³⁺ can be also used as a temperature sensor through the ratio of luminescence intensities recorded at two distinct time gates. Furthermore, the results confirmed that the phonon-assisted energy transfer process plays a dominant role in shaping the luminescence kinetics, surpassing the influence of the structural phase transition. Overall, this study highlights LiYO₂:Nd³⁺,Yb³⁺ as a promising candidate for multimodal luminescent temperature sensing applications.